The present invention relates to an electron gun for a color cathode ray tube, and more particularly to an in-line electron gun for a color cathode ray tube, having an improved focus electrode forming a quadruple lens.
An electron gun for a cathode ray tube is installed in a neck portion of a funnel connected to a panel and emits an electron beam to collide with a fluorescent layer. The general structure of the electron gun is as shown in FIG. 1.
A cathode 11, a control electrode 12 and a screen electrode 13 which constitute a triode, and a focus electrode 14 and a final accelerating electrode 15 which constitute a main lens, are sequentially formed in the electron gun. As shown in FIG.2A and FIG. 2B, focus electrode 14 is constituted by a first focus electrode 14a in which three vertically elongated electron beam passing holes 14H are formed on an outlet plane 14c, and a second focus electrode 14b in which three horizontally elongated electron beam passing holes 14H' are formed on an inlet plane 14d. A predetermined voltage is supplied to each electrode. A predetermined voltage Vf is supplied to first focus electrode 14a, and a dynamic focus voltage Vd (referenced to focus voltage Vf supplied to first focus electrode 14a) which is synchronous to a deflection signal, is supplied to second focus electrode 14b. Final accelerating electrode 15 receives an anode voltage Ve which is higher than focus voltage Vf.
As a predetermined voltage is supplied to each electrode, in a conventional color cathode ray tube constructed as above, a pre-focus lens is formed between screen electrode 13 and first focus electrode 14a of focus electrode 14, a quadrupole lens is formed between first focus electrode 14a and second focus electrode 14b in accordance with the supply of dynamic focus voltage Vd to second focus electrode 14b, and a main lens is formed between second focus electrode 14b and final accelerating electrode 15. Accordingly, when the electron beam emitted from cathode 11 scans the center of the screen, the quadrupole lens is not formed because there is no potential difference between first and second focus electrodes 14a and 14b, so that the electron beam emitted from cathode 11 passes through the main lens and then is landed on the center of the screen, Meanwhile, when the electron beam emitted from cathode 11 scans the periphery of the screen, a parabola-type dynamic focus voltage Vd synchronous to the deflection signal is supplied to the second focus electrode, so that a quadrupole lens is formed between first and second focus electrodes 14a and 14b, the electron beam emitted from cathode 11 is vertically elongated by the quadrupole effect when passing through the quadrupole lens. The vertically elongated electron beam passes through the main lens to be finally focused and accelerated, and then is deflected by the deflection yoke. Here, the electron beam is compensated so as to revert back to the circular shape by a non-uniform magnetic field, to scan the periphery of the fluorescent film. However, in such a conventional electron gun, a distorted electron beam is compensated by a non-uniform magnetic field of the deflection yoke, but since three in-line quadrupole lens are mutually interfered, a convergence characteristic is not good. Therefore, the resolution of a cathode ray tube having a conventional electron gun as above, becomes deteriorated.